Formation of CdS in Supramolecular DendrimerâDye Assemblies

Article pubs.acs.org/Macromolecules

Formation of CdS in Supramolecular Dendrimer−Dye Assemblies: Electrostatic and Electrostatic-Coordination Templating Jasmin Düring,† Benjamin Butz,‡ Erdmann Spiecker,‡ and Franziska Gröhn*,† †

Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials and ‡Institute for Micro- and Nanostructure Research & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander-University Erlangen-Nürnberg, 91058 Erlangen, Germany ABSTRACT: A novel concept for the formation of organic− inorganic hybrid nanostructures based on a tunable supramolecular templating mechanism involving several types of noncovalent interactions is presented. Different organic− inorganic hybrid structures with narrow size distributions composed of a cationic dendrimer, an oppositely charged divalent diazo dye, and CdS can be created: First, larger supramolecular structures consisting of several 100 to 1000 dendrimers is built based on ionic and π−π interaction. Second, with this self-assembled template, the hybrid nanostructure forms based on electrostatics and coordination interaction. The influence on shape and internal structure of the supramolecular hybrid architectures was investigated. A size control of the assemblies is possible in a range of 60−420 nm. The prospect to direct self-assembly and templating through the controlled combination of different noncovalent interactions potentially allows to create tailor-made systems for various applications.

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INTRODUCTION The combination of organic compounds with inorganic structures is of great interest for modern chemistry and material science.1−10 A prominent example from nature is dental enamel, which is composed of hydroxyapatite and proteins. Here, the combination of soft organic material and hard inorganic components on the nanoscale results in extraordinary stability and fracture strength. Also, in the fields of sensing or cancer therapy structures with organic and inorganic constituents are promising.11,12 An efficient route to produce such systems is self-assembly. Self-assembly takes advantage of noncovalent interactions such as hydrogen bonding, metal coordination, van der Waals, or ionic interactions. It is possible to introduce an inorganic compound into amphiphilic systems, such as micelles or vesicles,13−15 yielding interesting hybrid structures with potential applications in in vivo imaging or drug delivery.16 Choosing a semiconductor as inorganic compound opens the way to highly functional materials, for example in the field of solar energy conversion.17 It is, however, still challenging to control self-assembly in solution and gain stable aggregates of a defined size. With ionic organic building blocks, such as dendrimer macroions and dye counterions, we have shown that well-defined supramolecular assemblies can form by electrostatic self-assembly, owing to electrostatic and π−π interactions and the overall charge of the assemblies that equilibrate at a certain size.18−23 As “electrostatic nanotemplating” yields monodisperse inorganic nanoparticles inside of a polyelectrolyte template for dendrimers of higher generations,24−32 these two concepts have recently been combined to yield well-defined supramolecular hybrid © XXXX American Chemical Society

assemblies, both with noble metal nanoparticles such as gold and with semiconducting cadmium sulfide quantum dots.33 In that approach, the CdS was first prepared with the dendrimer macroions and subsequently the dendrimer−CdS particles were interconnected with an ionic aromatic diazo dye to larger supramolecular structures.33 Different approaches by other authors also resulted in versatile CdS-containing systems.35−42 Focus of the present study is the formation of CdS inside of preformed supramolecular assemblies other than of amphiphilic nature. This opens the way to new and versatile organic− inorganic hybrid structures. A great advantage of the system presented herein is that there is no necessity for specifically synthesized block copolymers, but a toolbox of different homopolymers and dyes may be used. In addition, the formation of a ternary macroion−dye−CdS structure bears potential in applications exploiting the functional combination of dye and semiconductor, e.g., in optoelectronics or solar energy conversion. To establish new types of organic−inorganic hybrid structures based on electrostatic self-assembly and to understand the influence of the components and exploit the interplay of interaction forces, different approaches are compared in this study. These rely to different extents on electrostatic and coordination interactions, which can be controlled by the order of the addition of building blocks. In addition, the effect of the ratios of the components, i.e., cadmium to dendrimer, dye to Received: May 29, 2015 Revised: November 7, 2015

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DOI: 10.1021/acs.macromol.5b01165 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Schematic Overview of the “Electrostatic Nanotemplating” Approaches A−C

L−1 in a volume of 3−4 mL was kept constant, and the concentrations of the other components varied according to the anticipated ratios. In approach C in the beginning the water had a pH of 11.5 and was after addition of G8 and Ar26 adjusted to pH = 3.5 with calculated amounts of HCl. The final switching to pH = 11.5 was conducted with NaOH. Light Scattering. The measurements were performed at an instrument equipped with a red HeNe laser (λ = 632.8 nm; 20 mW), an ALV 5000 correlator with 320 channels (ALV GmbH, Langen, Germany), and an ALV CGS 3 goniometer. The measurements covered an angular range of 30° ≤ θ ≤ 150°. The intensity autocorrelation function g2(τ) was for each angle transferred into the electric field autocorrelation function g1(τ) via the Siegert relation. The electric field autocorrelation function g1(τ) was successively transformed into the distribution of relaxation times A(τ) by a regularized inverse Laplace transformation using the program CONTIN developed by Provencher.32 From the distribution of relaxation times the apparent diffusion coefficient was calculated, and via extrapolation to zero scattering vector square the diffusion coefficient was obtained, with which via the Stokes−Einstein relation the hydrodynamic radius was calculated. The error bars were calculated from the error of the extrapolation to zero scattering vector square as described above.

dendrimer, and cadmium to sulfide, was investigated. It will also be discussed how far structures are thermodynamically or kinetically controlled.

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EXPERIMENTAL SECTION

Chemicals. The poly(amido amine) dendrimer of generation 8 (G8) was supplied by Dendritech, Midland, MI, and cadmium nitrate tetrahydrate (99%), sodium sulfide nonahydrate (98%), and Acid Red 26 (Ar26) were purchased from Sigma-Aldrich. The azo dye Ar26 was purified prior to use via recrystallization (96%), and the deionized water was filtered with two cellulose acetate membranes in a row, which had a pore size of 0.22 μm. Sample Preparation. Aqueous stock solutions of all chemicals were prepared without further adjustment of the pH value. The concentrations of the different stock solution were c(G8) = 5.3 × 10−6 mol L−1, c(Cd(NO3)·4H2O) = 3.2 × 10−3 mol L−1, c(Na2S·9H2O) = 4.1 × 10−3 mol L−1, and c(Ar26) = 2.0 × 10−3 mol L−1. The sodium sulfide stock solution was always prepared freshly before use. The samples were prepared by subsequently mixing water, G8, and the desired amounts of the other stock solutions in the desired order via stirring at 990 rpm. The final concentration of c(G8) = 2.0 × 10−7 mol B


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Macromolecules Scheme 2. Schematic Overview of the “Electrostatic-Coordination Nanotemplating” Approaches D and E

Figure 1. Investigation of a CdS−dendrimer−dye hybrid sample obtained through approach A. (a) Dynamic light scattering: electric field autocorrelation function g1(τ) and distribution of relaxation times A(τ) at a scattering angle of θ = 90°; width of distribution σ = 0.15, hydrodynamic radius RH = 116 nm. (b) UV−vis spectrum, where the spectrum of pure G8-Ar26 assemblies was subtracted. Via the tangent at the absorption onset the diameter of the corresponding CdS particles can be determined with the Henglein equation to d = 2.6 nm. (c, d) TEM pictures with (d) a zoom into the assembly of (c) and an average size of dTEM = 230 ± 88 nm (85 assemblies analyzed). rm(Ar26/G8) = 205, rm(Cd/G8) = 410, rm(S/Cd) = 1.5, rm(S/G8) = 614, and c(G8) = 2 × 10−7 mol L−1. UV−Vis Spectroscopy. Absorption spectra were recorded on a SHIMADZU UV spectrophotometer (UV-1800) with a slit width of 1.0 nm. For all measurements 10 mm quartz cuvettes were used. TEM. The transmission electron microscopy (TEM) images were acquired with a Zeiss EM 900 microscope, operated at 80 kV at magnifications ranging from 20000× to 250000×. For approach C

aberration-corrected HRTEM and STEM were performed using a FEI Titan3 80-300 microscope operated at 200 kV. The specimens were prepared by depositing 5 μL of the diluted sample solution onto carbon-coated copper grids, 300 mesh, and air-dry the grids. C


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Figure 2. Time-dependent investigation of (a) a G8−Ar26 sample after addition of Na2S (rm(Ar26/G8) = 204, rm(S/G8) = 614, starting 2 min after Na2S addition) and (b) dependence of the hydrodynamic radius RH on the time of addition of Cd2+ to the G8−Ar26−Na2S assemblies. The time is measured from the addition of Na2S solution to the addition of Cd2+solution (rm(Ar26/G8) = 204, rm(S/G8) = 614, rm(Cd/G8) = 410, rm(S/Cd) = 1.5, c(G8) = 2 × 10−7 mol L−1).

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RESULTS AND DISCUSSION Based on the connecting interactions that are responsible for self-assembly, the approaches can be separated into “electrostatic nanotemplating” and “electrostatic + coordination nanotemplating”. First, electrostatic nanotemplating where Na2S is added prior to Cd2+ will be discussed (Scheme 1, approaches A−C). Coordination interactions come into play when the Cd ions are added prior to the Na2S solution, which will be considered afterward (approaches D and E in Scheme 2). Electrostatic Nanotemplating. Approach A: G8-Ar26 plus Sulfide and Cadmium Ions. First, organic supramolecular structures with PAMAM dendrimer of generation 8 (G8) and the azo dye Acid Red 26 (Ar26) were formed in water at pH = 7.19 At neutral pH, the 1024 primary amine groups of the dendrimer macroion are charged, and therefore dendrimers become interconnected by the negatively charged dye due to ionic interactions and mutual π−π interactions of the dye molecules, resulting in defined supramolecular G8-Ar26 assemblies. Then a solution of Na2S was added. The sulfide is similar to the Ar26, 2-fold negatively charged and can therefore also interact with the dendrimer electrostatically. At last, a solution of Cd(NO3)2 was added to form CdS nanoparticles inside of the supramolecular structures. Dynamic light scattering, as shown in Figure 1, reveals a high size uniformity of the final hybrid assemblies with a very narrow distribution width of σ = 0.15 and a mean hydrodynamic radius of RH = 116 nm. The ratio of the components is denoted molar ratio rm and is defined as the number of molecules or ions to the number of the second kind of molecules or ions. As the dendrimer is a macromolecule with 1024 primary amine groups, which are charged at pH 7, charge neutralization equals at a molar ratio rm = 512, if all counterions are divalent. For the sample in Figure 1 the molar ratio of Ar26 to G8 is 205, leaving 614 excess primary amine groups. The molar ratio of Na2S to G8 is 614, resulting in an overall molar ratio of Ar26+S2− to G8 of 819. This translates into an overall charge ratio of 1.8, i.e., an over-stoichiometric charge ratio. UV−vis spectroscopy can be used to analyze the CdS nanoparticles. After subtracting the UV−vis spectrum of pure G8-dye assemblies from the UV−vis spectrum of the hybrid sample, the characteristic band for CdS is visible, proving the

existence of nanoscale CdS (Figure 1b). The corresponding diameter of the nanoparticles can be determined with the Henglein equation and is 2.6 nm.44−46 As the luminescence of CdS is quenched by Ar26,33 emission spectroscopy could not be used to analyze the CdS nanoparticles. In the TEM measurements, as can be seen in Figure 1c,d, spherical supramolecular aggregates are visible with CdS particles in the inside. The size of the CdS particles is about 2−3 nm. This fits well with the UV−vis result. The size of the assemblies is also in accordance with DLS measurements (RH = 116 nm, i.e., DH = 232 nm, dTEM = 230 ± 88 nm, 85 assemblies analyzed). Additionally, some CdS particles can also be found that are not within the assemblies, but in the inside of nonaggregated dendrimers (Figure 1c). Further, it is interesting to consider the formation of the hybrid particles: The S2− ions can connect the G8 dendrimers similar to the Ar26 via electrostatic interactions. Therefore, the G8−Ar26 assemblies are expected to increase in size with increasing molar ratio of rm(S/G8). Yet, what is observed is that the G8−dye assemblies dissolve completely upon addition of Na2S and then start to form again with time, to build even larger, still spherical aggregates. The dissolution of the G8− Ar26 aggregates is probably caused by an increase in pH upon addition of the basic solution of Na2S (pH = 6.9 (G8−Ar26) to pH = 8.8 (+Na2S)). This pH increase causes a partial deprotonation of the primary amine groups of the dendrimer (25.4% protonation left). Therefore, the aggregates that are stabilized by electrostatic and π−π-interactions dissolve. It could be shown that the pH decreases with time to pH = 7.8 (35.9% protonation), and therefore the electrostatically stabilized aggregates form again. The decrease in pH may be caused by the equilibrium between S2−, HS−, and H2S.47,48 After the dissolution of the assemblies, the pH is still sufficiently low so that about 25% of the charges remain at the G8 dendrimer. Thus, S2− and HS−can, at least to some extent, interact electrostatically with the primary amine groups. This influences the balance between S2−, HS−, and H2S and causes the slow release of H+ into the solution. Hence, the pH of the solution decreases again, facilitating the renewed formation of assemblies, which is in accordance with the observations. The aggregates start to form again after a few minutes and grow until they reach a final size after about 90 D


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Macromolecules min, as can be seen in Figure 2a. The G8−Ar26−Na2S assemblies with RH = 211 nm are larger than the original G8− dye aggregates (RH = 65 nm), which is reasonable, as the S2− also contributes to the interconnecting electrostatic interactions additionally to Ar26 (rm(Ar26/G8) = 205 and rm(S/G8) = 614). After the addition of Cd2+ and the so-caused formation of CdS particles, the growth is stopped and the aggregates are stable. The final assemblies are about 10% smaller than the Na2S−G8−Ar26 assemblies. The size decrease can be understood as S2− is taken out of the equation due to the formation of CdS. Thus, the number of interconnecting particles decreases resulting in smaller supramolecular structures. Upon addition of Cd2+ solution to the sample at a certain time during the growth process, the size of the resulting organic−inorganic hybrid structures can be tuned. The molar ratio of Cd/G8 does not seem to be that decisive, as for different ratios of Cd/G8 the same assembly sizes result (tested in a range of rm(Cd/G8) = 410−1178). The only size determining factor is the time of the addition of Cd2+ (Figure 2b). Approach B: G8-Sulfide Plus Ar26 and Cadmium Ions. In this approach the Na2S was added first to the G8 dendrimer to form S2− loaded dendrimer through electrostatic interactions. This loaded dendrimer was then interconnected with Ar26 to form supramolecular assemblies, and finally CdS was precipitated inside of the structure by the addition of Cd(NO3)2 solution. This means experimentally the only difference of approach B compared to approach A is the order of addition of Na2S and Ar26. Dynamic light scattering reveals that with increasing molar ratio of Na2S to dendrimer the resulting sizes of the aggregates increase (Figure 3). On the

lcharge =

c(counterion)· number of charges c(dendrimer) ·number of charges

On the other hand, the HS− or S2− ions probably occupy primary amine groups of the dendrimer additionally to the Ar26, which results in higher overall counterion concentration and therefore a higher molar ratio and larger structures. After formation of CdS via the addition of Cd(NO3)2 the aggregates show only minimal size changes as can be seen in Figure 3. In TEM measurements (Figure 4) spherical assemblies are visible with an average diameter of dTEM = 204 ± 59 nm (50 particles analyzed). This is in good agreement with the results of the DLS measurement (RH = 91 nm). At higher magnifications a structure is visible inside of the aggregates. These architectures are of the same size as single G8 dendrimer (11 nm). Because of the high excess of Cd2+ (rm(Cd/S) = 1.9), the dendrimer molecules likely are stained and therefore visible as gray shadows. The UV−vis spectrum of this system (not shown) confirms the presence of CdS nanostructures, yet the onset of the absorption band is not as steep as in approach A, which means that the CdS is less monodisperse in this approach. The most important difference between these two systems is that in approach A the variation of the assembly size can be achieved by altering the time of Cd2+ addition, whereas in approach B the assembly size can be adjusted with variation of the molar ratio of S to G8. Both systems result in spherical monomodal assemblies with CdS nanoparticles inside of the structures. Approach C: Hydroxide Switching. To minimize the influence of the pH on the size, in approach C the CdS was formed in acidic medium. This means that at first the supramolecular G8−Ar26 aggregates were formed by mixing G8 and Ar26 at high pH (≥ 10), and then the system was “switched on” by reducing the pH below 4. This method of supramolecular assembly formation (without CdS) is well established and understood.28 Then a fresh Na2S solution was added to the G8−Ar26 assemblies to enable the HS− or S2− to enter the supramolecular assemblies due to electrostatic interactions. After addition of Cd(NO3)2, CdS forms. To stabilize the freshly formed CdS, the pH was adjusted to above 10. Here the primary amine groups are no longer charged. The electrostatic interactions between the dendrimer and the dye molecules are ceased, and the supramolecular aggregates may be expected to dissolve. In the UV−vis spectrum (Figure 5a) the disappearance of the π−π-stacking of the dye molecules can be observed,27 which means that the Ar26 is released into the solution and is no longer bound to the dendrimer. Nevertheless, with light scattering well-designed structures can be detected, both before and after addition of NaOH, as can be seen in Figures 5b and 5c. This result can only be understood if the CdS holds the G8 together. Either the Cd2+ ions, which are in excess as compared to the S2−, coordinate to the primary amine groups and interconnect them thereby, or the CdS itself functions as “glue”, interconnecting the G8 macromolecules physically and prevents the dendrimers from disassembling. Also, the Hamaker attraction between the CdS colloids might play a role. Interestingly, when S2− is in excess, the aggregates become much smaller after switching (from 40 to 20 nm), and visible precipitate forms. Shortly after the preparation the sample precipitates. Therefore, all samples in the following are prepared with an excess of Cd2+ (usually rm(Cd/S) = 2). A

Figure 3. Approach B: dependence of the hydrodynamic radius RH on the molar ratio of S to G8 before (black) and after addition of Cd2+ (red). With increasing amount of S2− larger aggregates are formed, whereas the sizes stay constant after addition of Cd2+. Sample with rm(Ar26/G8) = 205, rm(Cd/S) = 1.5, c(G8) = 2 × 10−7 mol L−1, and averaged width of distribution σ = 0.16.

one hand, this might be due to the pH, which increases with increasing amount of Na2S. With higher pH the dendrimer is less charged, which causes a higher effective charge ratio of Ar26 to G8 and therefore larger supramolecular aggregates. Here the charge ratio is defined as the molar number of charges of the counterion to the molar number of charges of the dendrimer: E


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Figure 4. Approach B: TEM pictures of G8−Ar26−CdS hybrid assemblies prepared with approach B. (a) Overview over several assemblies, (b) magnification of a single assembly, (c) zoom into an assembly; average assembly size dTEM = 204 ± 59 nm (83 assemblies analyzed). Sample: rm(S/ G8) = 276, rm(Ar26/G8) = 205, rm(Cd/G8) = 522, and rm(Cd/S) = 1.9.

Figure 5. UV−vis and dynamic light scattering investigation of samples prepared via approach C: (a) UV−vis spectra in acidic medium (black curve) displaying the dye π−π stacking and after addition of NaOH (blue curve), indicating the disappearance of the π−π stacking (rm(Ar26/G8) = 717, rm(Cd/S) = 2, rm(S/G8) = 512); (b, c) DLS electric field autocorrelation functions g1(τ) and distributions of relaxation times A(τ) (scattering angle θ = 90°): (b) G8−Ar26 assemblies at pH 3.5 and (c) CdS−G8−Ar26 hybrid assemblies at pH 11 with the small peak in both cases corresponding to single dendrimer molecules (rm(Ar26/G8) = 717, rm(Cd/G8) = 1054, rm(Cd/S) = 2.0, rm(S/G8) = 526, c(G8) = 2 × 10−7 mol L−1). (d) Dependence of the hydrodynamic radius RH on the molar ratio of S to G8. The blue line indicates the size of the pure G8−Ar26 assemblies at pH 3.5 without present CdS (rm(Ar26/G8) = 922, Cd2+ in excess, c(G8) = 2 × 10−7 mol L−1).

For rm(S/G8) = 256, the aggregates are smaller (29 nm), while for rm(S/G8) = 512 the size stays constant (46 nm). At a higher molar ratio (rm(S/G8) = 1536), the aggregates are larger than before (59 nm). This can be understood by the different amounts of CdS that forms at different molar ratios. For small

molar ratio of rm(Ar26/G8) = 922 (90% of the amine groups occupied) results in supramolecular aggregates with RH = 44 nm. After addition of Na2S and Cd(NO3)2 and increase of the pH, different sizes can be found for different molar ratios of S to G8, as depicted in Figure 5d. F


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Figure 6. TEM pictures of G8−Ar26−CdS hybrid assemblies prepared with approach C: (a) overview over several assemblies, (b) magnification of single assemblies, and (c) STEM image of one assembly, where the outer shell and the interior structure of the CdS is noticeable. (d) HRTEM image of a similar assembly with visible atomic planes of CdS. Average size dTEM = 39 ± 9 nm (78 assemblies analyzed). Sample specifications: rm(S/G8) = 512, rm(Ar26/G8) = 717, Cd2+ in excess.

microscopy (STEM) images, only small dark “channels” can be found, meaning that only little organic material is present inside of the “CdS domain”. The high-resolution TEM (HRTEM) measurements reveal that the CdS structure is not monocrystalline. This suggests that the CdS forms inside of the dendrimer and then interconnects upon further growth, forcing the organic part of the assembly to the exterior. Thus, with this approach anisotropic CdS can be formed in a new hybrid particle by interconnection of CdS nanoparticles upon growth within the confined space of the assembly. Although the order of addition of the single components is the same as in approach A, completely different assemblies with larger and more complex CdS structures result in this approach, showing the intense influence of the pH on the resulting hybrid structures. In approach A at neutral pH only the outer shell of the dendrimer is charged and therefore accessible for the counterions, whereas in the present approach C, at initially low pH, also the inner amine groups are charged. For that reason the dendrimer can also be loaded in the interior and with a much higher number of counterions, still resulting in stable aggregates. This approach C is reminiscent of a previously investigated gold−microgel system.22 The microgel serves as a nanoreactor and influences the growth of various gold nanostructures in its interior, such as small spheres and anisotropic nanonets and nanonuggets. The gold is monocrystalline despite the curvature of the structures, indicating a conducted growth process. This is in stark contrast to approach C here, where the CdS is polycrystalline due to nucleation in dendrimer templates and interconnection of the CdS particles upon further growth inside of the dendrimer−dye assembly. Yet, both systems utilize the confined space of an organic nanoreactor (microgel or dendrimer−dye assembly) to form exceptional unusual inorganic nanostructures.

ratios less CdS can precipitate inside of the supramolecular aggregates, which is not sufficient to hold the aggregate together. Hence, a certain part of the G8 molecules dissolves, and the rest is “glued” together by the CdS. At rm(S/G8) = 512 the size stays nearly constant because the amount of CdS is high enough to stabilize the whole aggregate. At higher molar ratios there is so much excess of Cd2+ present in the solution that additional aggregation is caused, and therefore the aggregates grow and finally precipitate after a few hours. A sample with rm(Ar26/G8) = 717, rm(S/G8) = 512, and rm(Cd/S) = 2.0 was investigated with TEM. In the TEM images, as shown in Figure 6, small dark dense spherical assemblies are visible. At higher magnification CdS particles of 3−5 nm size are observable within the assemblies, which are grown together or lie in very close proximity to each other. An organic shell with a much lower contrast surrounds the assemblies. This organic shell might be the reason why the aggregates have only a diameter of around 40 nm in TEM, while with DLS a diameter of 70 nm is detectable. At low pH of 3.5 all 2046 amine groups of the G8 dendrimer are charged. Therefore, the Na2S can permeate the entire G8 dendrimer before the CdS is precipitated in the inside of G8 by addition of Cd2+. CdS is not stable in low pH medium, yet it was found that it can indeed form at low pH (3−4), but it is not stable for more than a few hours.49 Because of the higher molar ratio of S to G8, which is accessible at this low pH value, more S2− is present inside of the dendrimer, and therefore the resulting CdS particles are larger than in the previously investigated systems at pH 7. The analysis of UV−vis absorption measurements indicates an interaction of the dye with CdS. Thus, the analysis of the CdS size via Henglein equation has to be treated with caution but yields an approximate size of 4.8 nm, which corresponds well with the entities visible in TEM. In scanning transmission electron G


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free spherical G8−Ar26 assemblies (dTEM = 194 ± 40 nm, Figure 8e). For the higher concentration of Na2S, the CdS aggregates are much larger, which is in accordance with the light scattering results (Figure 8a,d). These findings can be understood in that way that the Cd2+, which is added to the G8−Ar26 assemblies, coordinates predominantly to the free and unbound G8, which coexists with the assemblies.20 This is so because the Ar26 inside of the assemblies shields the primary charged amine groups. Therefore, the coordination of Cd2+ to primary amine groups inside of the assembly is hindered, and thus, only in the outer section of the assemblies CdS particles can be found. After the addition of Na2S the free dendrimer loaded with Cd2+ becomes interconnected by the forming CdS, resulting in two kinds of supramolecular species (G8−Ar26 assemblies and anisotropic CdS containing aggregates), which are roughly in the same size range. For DLS, the two species are too close together and cannot be separated, and the resulting peak averages both sizes, yielding a smaller overall radius. This is also confirmed by the distribution width, which is very narrow for the pure G8−Ar26 sample (σ = 0.07) and with σ = 0.18 slightly wider for the final hybrid sample. The same understanding applies to samples with higher Na2S concentration and larger assembly sizes. The widths of distribution (σ) for different molar ratios are elucidating the broadening of the DLS signal up to σ = 0.34 for rm(S/G8) = 593 after addition of CdS in comparison to the initial very monomodal G8−Ar26 assemblies (all σ = 0.07 for rm(Ar26/G8) = 205). Next it is of interest whether the size of the assemblies can be tuned by the variation of the molar ratio rm(Ar26/G8). In accordance with previous systems, it was found that with increasing rm(Ar26/G8) the assemblies become larger.19 The subsequent addition of Cd2+ and Na2S does not change the size much. Only at rather high rm(Ar26/G8) = 460, the sample precipitates after addition of Cd2+ and S2−. As can be seen in Figure 9, the radii of the assemblies can be tuned in a range of 30−160 nm. With TEM again spherical assemblies without CdS and smaller anisotropic structures with distinct CdS particles are visible (Figure 9b). As a sample with rm(Ar26/G8) = 256 was investigated here, the spherical organic assemblies are slightly larger than at rm(Ar26/G8) = 205 (193 nm instead of 132 nm). The CdS containing aggregates have an average size of 49 nm. It can be concluded that with increasing rm(Ar26/G8) the organic assemblies become larger, whereas the CdS containing hybrid particles become smaller, as less free dendrimer is left. Thus, the results fit well with the given explanation. Comparing this approach to approach A elucidates the influence of the kind of interaction between the different building blocks on the resulting structure. Here the coordination of Cd2+ to primary amine groups is the decisive interaction rather than ionic interactions between the negatively charged S2− and the positive charges of the dendrimer. Therefore, in approach D two different kinds of structures can be found, namely organic G8−Ar26 spheres and anisotropic CdS hybrid aggregates, where the anisotropic assemblies are formed by the coordination interaction of Cd2+ to primary amine groups of free dendrimer. Approach A resulted in only spherical assemblies with CdS inside of the structure, as the coordination interaction of the cadmium does not play a role in that approach. Approach E: G8−Cadmium plus Ar26 and Sulfide Ions. In approach E, the Cd salt was mixed with G8 first, in order to

Electrostatic-Coordination Nanotemplating. Approach D: G8−Ar26 plus Cadmium and Sulfide Ions. In this approach the G8−Ar26 assemblies were formed at pH 7 (like in approach A), but then the Cd salt was added prior to Na2S. Unlike S2− and Ar26, the Cd2+ does not interact with the positive charges electrostatically but coordinates to the primary amine groups.32,50 A fresh solution of Na2S was then added to “precipitate” the CdS within the dendrimer−dye assemblies. For the further consideration the excess molar ratio rm,excess(S/G8) is defined as the number of sulfide per dendrimer that is left after the precipitation with Cd: rm,excess(S/G8) =

c(S2 −) − c(Cd2 +) c(G8)

The initial G8−Ar26 assemblies do not show any size response to the addition of Cd2+ and stay at a size of about RH = 65 nm for different Cd-to-dendrimer ratios. At a fixed molar ratio of rm(Cd/G8) = 410 and an excess of sulfide (rm,excess(S/G8) > 410), the size of the aggregates, as measured with DLS, changes upon addition of Na2S (Figure 7). With increasing amount of

Figure 7. Approach D: dependence of the hydrodynamic radius RH of the hybrid assemblies on the excess molar ratio of S to G8. The red curve indicates the size of the pure G8−Ar26 assemblies independently from the x-axis, whereas the black curve shows the sizes of the final assemblies with CdS. Where not stated otherwise in the graph, the errors were < 1.4 nm. Sample: rm(Ar26/G8) = 205, rm(S/Cd) = 2, c(G8) = 2 × 10−7 mol L−1.

Na2S the size initially decreases slightly from about RH = 65 nm to minimum RH = 52 nm until, at a certain excess molar ratio rm,excess(S/G8), the radius increases to about 110 nm, and the aggregates are anisotropic in shape. At higher amounts of Na2S the samples start to precipitate. The large impact of small changes in preparation conditions on the resulting structures might indicate a kinetic control of the system. These results can again be understood based on TEM images. Figure 8 shows TEM pictures and associated DLS data of samples with excess molar ratio rm,excess(S/G8) = 302 (a, b, and c) and rm,excess(S/G8) = 430 (d, e, and f), both rm(Cd/G8) = 410, c(G8) = 2.0 × 10−7 mol L−1. At the lower concentration of Na2S only few CdS particles (≈ 3 nm) are visible inside of the larger spherical assemblies (dTEM = 132 ± 22 nm, Figure 8c). Most of the particles are located in smaller assemblies, which tend to be anisotropic (average diameter dTEM = 48 ± 22 nm). At a higher Na2S concentration, again most of the CdS particles can be found in anisotropic assemblies (average diameter dTEM = 221 ± 89 nm) that coexist with rather CdS H


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Figure 8. Investigation of two G8−Ar26−CdS hybrid samples prepared trough approach D: (a−c) sample with rm,excess(S/G8) = 302, (d−f) sample with rm,excess(S/G8) = 430. (a, d) Dynamic light scattering autocorrelation functions with distributions of relaxation times (scattering angle θ = 80° and θ = 90°); (b) TEM overview of several assemblies with dTEM(spherical assemblies) = 132 ± 22 nm and dTEM(anisotropic assemblies) = 48 ± 22 nm; (c) displays a higher magnification of one larger and several smaller assemblies; (e) anisotropic CdS-containing assembly; (f) purely organic assemblies in coexistence with anisotropic CdS containing aggregates with average sizes of dTEM(spherical assemblies) = 194 ± 40 nm and dTEM(anisotropic assemblies) = 221 ± 89 nm; all: rm(Ar26/G8) = 205.

Figure 9. (a) Dependence of the hydrodynamic radius RH on the molar ratio of Ar26 to G8 for pure G8−Ar26 assemblies (black curve) and after formation of CdS (red curve); where not depicted in the graph, the error was < 2.5 nm; sample: rm(Cd/G8) = 410, rm(S/Cd) = 1.5, c(G8) = 2 × 10−7 mol L−1. (b) TEM of a sample with rm(Cd/G8) = 410, rm(S/Cd) = 1.5, rm(Ar26/G8) = 256, and an average size of dTEM(spherical assemblies) = 193 ± 50 nm and dTEM(anisotropic assemblies) = 49 ± 23 nm (at least 80 particles analyzed).

achieve Cd-loaded G8 dendrimers, which then are interconnected with Ar26. Thus, after addition of Na2S the CdS may be expected to be inside of these G8−Cd−Ar26 hybrid assemblies. With this approach the presence of two different supramolecular species, as it is the case in approach D, should be prevented.

In Figure 10a,b, TEM images of a chosen sample of this approach are depicted. Only one type of supramolecular species has formed, which is anisotropic in shape. The CdS is located in the interior of the structure and forms crystalline nanoparticles. UV−vis spectra indicate particles of the size of 2.4 nm, which is in good agreement with the TEM images. I


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Macromolecules

Figure 10. TEM of G8−Ar26−CdS hybrid samples prepared trough approach E: (a) rm(Ar26/G8) = 205, overview of several anisotropic assemblies, (b) rm(Ar26/G8) = 205, larger magnification with single CdS particles being visible. (c, d) rm(Ar26/G8) = 307 with the CdS less densely packed inside of the supramolecular hybrid structures than in (a) and (b). (c) A single hybrid assembly; (d) a larger magnified section of one assembly, where the single CdS particles are clearly visible. rm(Cd/G8) = 205, rm(S/Cd) = 2.

Figure 11. Approach E: (a) Dependence of the hydrodynamic radius RH on the molar ratio of Cd to G8 before addition of Na2S (red curve) and afterward (black curve) (always rm(S/Cd) = 1.5 and rm(Ar26/G8) = 205). The arrow indicates the molar ratio at which maximal coordination of the Cd2+ to the primary amine groups of the G8 is reached. Unless illustrated in the graph, the error was < 1.7 nm. (b) Dependence of the hydrodynamic radius RH on the molar ratio of Ar26 to G8. The red curve corresponds to the assembly sizes G8−Cd2+−Ar26, the black curve shows the final sizes after the addition of Na2S. Unless depicted in the graph, the error was in the range of 0.5−7.3 nm. rm(Cd/G8) = 205, rm(S/Cd) = 2, rm(S/G8) = 410, and c(G8) = 2 × 10−7 mol L−1.

For a better insight into the system the behavior of the hydrodynamic radius of the G8−Cd2+−Ar26 assemblies has

been examined. In contrast to the previous approach D, where no influence of Cd2+ on the size was found, the hydrodynamic J


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Macromolecules radius of the G8−Cd2+−Ar26 assemblies decreases slightly with increasing amount of Cd2+, as can be seen in Figure 11 (red curve). At low molar ratios rm(Cd/G8) the radius is similar to the pure G8−Ar26 assembly (60 nm compared to 65 nm without Cd2+). Then the radius decreases gradually to 34 nm and stays about constant for molar ratios rm(Cd/G8) > 256. This number corresponds to the maximum amount of Cd2+ that can coordinate 4-fold51,52 at the outer shell of the G8, i.e., at the 1024 primary amine groups. A reason for this behavior could be that the Cd2+ causes the charged primary amine groups to release their protons upon coordination with the Cd2+.53−55 This is indicated by a decrease of the pH upon addition of Cd2+ to the G8 (from pH = 8.5 to pH = 7.9). Because of the Cd2+, the surface of the G8 is still positively charged and can therefore be interconnected by the negatively charged Ar26 electrostatically. Yet the assemblies are with higher molar ratio of Cd2+ not as well-defined as with smaller amounts of Cd2+ ((Cd/G8) = 102: σ = 0.05 and (Cd/G8) = 614: σ = 0.29), which might indicate a change in interconnecting interactions or geometric effects. It is reasonable that the released protons (at least) partly protonate the tertiary amine groups,56 resulting in a higher overall charge of the dendrimer. As the number of charges is the crucial factor in electrostatic interaction, a higher charge of the dendrimer at constant amount of Ar26 changes the ratio of charges of Ar26 to G8. The effective charge ratio of Ar26 to G8 is lower, thus causing smaller structures. For molar ratios rm(Cd/G8) > 256 the sizes of the assemblies stay constant, as all primary amine groups are coordinated to Cd2+ and cannot release protons anymore. Only the amount of free Cd2+ in solution increases. This has only the influence on the assemblies, as that the distribution widths become larger, thus the assemblies are no more as well-defined as with smaller loading ratios of Cd2+. In analogy to approach D (G8 + Ar26, then Cd2+) the final assembly size is dependent on the loading ratio of excess Na2S solution. In this system the Na2S only has an influence on the resulting structures when the excess molar ratio of Na2S to G8 is larger than 154. For small molar ratios of Na2S the size is nearly identical to the assemblies before addition of Na2S. Only when the excess molar ratio is larger than 154, the size significantly increases after the precipitation of CdS. Yet the assemblies do not show a clear dependence on either the molar ratio of Cd to G8 or on the excess molar ratio of Na2S to G8. A reason for this might be that the assemblies are kinetically controlled. This means that very small changes in ambient conditions at the time of sample preparation, which are very difficult to reproduce, can have a great effect on the resulting structures. Nevertheless, the amount of added Na2S influences not only the size of the resulting assemblies but also their shape. The ratio of the radius of gyration to the hydrodynamic radius, which gives an indication of the shape of the structure, increases with increasing amount of Na2S. This can be interpreted in the way that the shape of the aggregates is more elongated with a higher amount of Na2S. TEM measurements also confirmed this theory. Upon variation of the molar ratio of Ar26 to G8 (in the presence of constant amounts of Cd2+), the size of the resulting aggregates increases with increasing amount of Ar26 (red curve in Figure 11b). After the addition of constant amounts of Na2S (rm(Na2S/G8) = 205) (black curve in Figure 11b), the sizes of the assemblies become larger for larger molar ratios of Ar26 to G8. At low molar ratios (205 and 256) the size of the

assemblies stays constant after addition of Na2S. Only when the molar ratio rm(Ar26/G8) reaches 307, the sizes of the assemblies increase after addition of Na2S, probably because the combined molar ratios of Ar26 and S2− (both 2-fold negatively charged) then exceed 512 (307 from Ar26 + 205 from S2−); i.e., all primary amine groups are occupied. Such a high molar ratio affects the assembly sizes much stronger than understoichiometric ratios, causing therefore a stronger size increase and growth of the assemblies over time.18 A sample with rm(Ar26/G8) = 307 was measured with TEM (Figure 10c,d). Slightly anisotropic assemblies with CdS inside of the structures are visible (assembly sizes about 300 nm, CdS about 2.5 nm in diameter). The single CdS particles are packed less densely than in samples with lower Ar26 content (cf. Figure 10b). This is because in this case more organic material is present with the same amount of CdS, resulting in assemblies with less CdS content. Therefore, the single CdS particles lie farther apart from each other and are not as densely packed as in the case of lower Ar26 content (e.g., rM(Ar26/G8) = 205, Figure 10a,b). Hence, the number density of CdS nanoparticles inside of the assembly can be controlled by the amount of dye, yielding higher number densities for low amounts of dye and lower number densities of CdS nanoparticles for higher amounts of dye. By comparing the last two approaches, one can see that even slight differences as changing the order of addition of Cd2+ and Ar26 results in different supramolecular structures. The CdS particles, on the other hand, look roughly the same in both approaches, as the size is only controlled by the microenvironment of the G8 dendrimer, which is the same in both systems. Another similarity is that both systems appear to be kinetically controlled, in contrast to those approaches where Na2S was added prior to Cd2+. Comparative Discussion. It was shown that even with identical material a variety of different hybrid nanostructures is accessible. In the electrostatic nanotemplating approaches A− C, when Na2S was added prior to Cd2+, the systems are thermodynamically controlled. The structures are spherical and very well-defined, and the CdS can be found in the interior of the supramolecular assemblies. In the electrostatic-coordination nanotemplating approaches D and E, on the other hand, the systems appear to be kinetically controlled, which is indicated by the strong dependence on the preparation conditions. The resulting structures are anisotropic, and the sizes can, to some extent, be controlled by the variation of the molar ratios of S to G8. Also, the number density of CdS nanoparticles inside of the assemblies can be tuned over the ratio between Ar26 and G8 in approach E. The size of the CdS nanoparticles can be controlled in a very small range. In all approaches at pH = 7, the CdS has a favored size of 2−3 nm. In approach C, where the assembly formation occurred at pH < 4, the CdS nanoparticles have a size of 3−5 nm. Additionally, they might even be grown together or are at least densely packed inside of the supramolecular structures. Therefore, the whole assembly is hindered from disassembling by Hamaker interaction, by the interconnection of the CdS particles or through the physical interconnection of the G8 molecules by the CdS. Thus, with this study we have elucidated the impact of binding motives and routes on supramolecular self-assembly. The concept of CdS preparation with dendrimer as stabilizing agent has been well-known.25,29,30,32,33,41 Different CdS containing structures have been obtained by self-assembly mechanisms, such as hybrid Langmuir monolayers,57 self-assembled films with CdS K


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Macromolecules and polyallylamine hydrochloride (PAH),58 and one-dimensional CdSe−polyelectrolyte nanofibers.59,60 Here, the recently established self-assembly properties of dendrimer with the dye Ar2618−21 were combined with the ability of dendrimer to stabilize CdS to construct new hybrid CdS containing assemblies.

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CONCLUSION In this study a hybrid material was investigated, which is composed of cationic PAMAM dendrimer of generation 8 (G8), the anionic dye Acid Red 26 (Ar26), Cd(NO3)2, and Na2S forming CdS nanoparticles. We could show that different organic−inorganic hybrid structures with narrow size distributions can be produced with the same building blocks in aqueous solution by deliberate choice of the templating mechanism. Key for controlling the structure is a delicate interplay of different noncovalent interactionselectrostatics, coordination, π−π, and Hamaker interactionwhich can be tuned through the order of addition of the building blocks, the ratio of the components, the time of addition of certain chemicals, and the pH. Different structures such as monomodal spherical CdS containing assemblies (approach A + B), spherical purely organic assemblies coexisting with anisotropic CdS containing hybrid particles (approach D), solely anisotropic hybrid structures with 2.5 nm CdS inside (approach E), and small dense spheres composed of anisotropic CdS with entities in the size range of 3−5 nm, Ar26 and G8 (approach C) are accessible. Altogether, size-tuning of the hybrid assembly is possible in a range of 60−420 nm. With the comprehension of the complex correlations of this system an expansion to different inorganic and/or organic building blocks, such as different dendrimer generations, is also feasible. Thus, we could show the facile access to versatile supramolecular structures obtained by self-assembly with a small stock of building blocks. This allows for custom tailoring of structures, explicitly designed for various applications that benefit from organic− inorganic hybrid structures, such as solar energy conversion or optoelectronics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of Deutsche Forschungsgemeinschaft (DFG), the Interdisciplinary Center for Molecular Materials (ICMM, University Erlangen-Nürnberg), and Solar Technologies go Hybrid (SolTech) is gratefully acknowledged. We also thank the Cluster of Excellence Engineering of Advanced materials (EAM) of the Deutsche Forschungsgemeinschaft (DFG).

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REFERENCES

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